Our prior applications disclose manufacture of contact and intraocular lenses by forming reference surfaces in a lens blank along with the posterior surface for the lens. The partially formed lens blank is then mounted to a blocking member which has complementary surfaces that mate with the reference surface and automatically align the lens blank. The blocking member has a flange that abuts against a spindle or collet in which the member is held during formation of the outer lens surface. The known distance between the abutting surface of this flange and the reference surface on the lens blank allows the front surface of the lens to be formed without having to separately measure the thickness of the lens blank, or enter dimensional information into the computer which controls the operation. A recessed collet within the spindle enables the blocking member flange to directly abut the end of the spindle.
The blocking member for forming the front surface curve was preferably formed of plastic in order to preserve tool life of the diamond cutting tool. The reference ledges were cut away during cutting of the front surface of the lens. It was suggested that the front surface of the blocking member could be reshaped and reused after the front surface of the lens was cut and the annular cylindrical ledges were cut away. The blocking member was manufactured in a two part mold.
The axis of the front curved surface was not reliably concentric with the outer axis of the blocking member. One proposal is to produce a short front curve blocking member having a length which is less than 1/2 of the outer diameter of the member. However, the short blocking member does not have a mounting shank and must be grasped in the collet of a spindle. Slight variations in the external diameter can affect the position of the block in the collet and can cause variations among lenses produced in a blocking member even if the base curves of the blanks have a constant center thickness. The short blocking member is usually made of plastic. Again, the force of the collet grasping the block can distort its shape and cause variations in the position of the reference ledge and the shape of the surface complementary to the base curve.
Different lenses require different posterior surfaces. This requires manufacture and inventory of blocking front curve members having a range of curves. In the case of contact lenses, base curves with radii of 7, 8 and 9 mm are usually required. This increases the complexity of molding the blocking members, requires an inventory of many small parts and can lead to human error in selection of the correct blocking member.
Moreover, the front curved, convex surface of the molded plastic blocking member can also have variations from shrinkage of the resin during cure and from imperfections in the surface of the mold cavity. If the front surface blocking member has a metal surface, it can be polished to a much more perfect finish and is hard and does not deflect. However, as discussed above, the need to cut the ledge surrounding the curved surface of the blocking member makes the use of a metal block impractical for supporting the base curve while cutting the front curve.